Mobile telecommunication systems provide communications among mobile units and between mobile units and land-based customer stations using a limited number of radio channels. In the Advanced Mobile Phone Service (AMPS) System described in the Bell System Technical Journal, (BSTJ), V. 58, No. 1, pp. 1-269, Jan., 1979, a mobile unit is served by a different radio station, sometimes referred to as a cell site, as it moves from one cell area to another in a region. The radio stations are connected to a controlling mobile telecommunications switching office (MTSO) that provides access to land-based customer stations via the common-carrier telephone network. The cellular arrangement of radio stations makes it possible for distant mobile units to re-use the same radio channels without interference. To prevent a mobile unit from traveling so far out of one cell area that its signal could interfere with communications in a remote cell area using the same radio channel, the signal strength of a mobile unit is monitored by the serving radio station. A "vehicle location" operation is initiated when the signal strength falls sufficiently low to indicate that the mobile unit is leaving the boundary of the cell area.
When the strength of the signals from the mobile unit drop below a threshold value, the serving radio station notifies the MTSO. The MTSO then requests nearby radio stations to measure the received signal strength of the mobile unit. In response to the received signal strength data from the group of nearby radio stations, the MTSO selects the radio station which would best be capable of taking over control of communications with the mobile unit and a "hand-off" of control is made to that radio station.
With a limited number of radio channels and a growing number of mobile units to serve, it is necessary to re-use channels more frequently. This requires a decrease in the size of each cell area and/or a splitting of cell areas into sectors each sector associated with a different directional antenna; in either case, more frequent vehicle location and hand-off operations are required. Each such vehicle location and hand-off requires an intensive usage of the data processing facilities of the MTSO. As an MTSO serves more mobile units, the use of these data processing facilities increases both with the number of calls and with the greater number of hand-offs required per call. Ultimately, the data processing load on an MTSO could be expected to approach saturation. Because the use of multiple MTSO's in one region would be expensive and inefficient it would be advantageous to be able to permit an increase in the number of vehicle locations and handoffs processed by a system without greatly increasing the data processing load on the MTSO.